The present invention relates to a process for removing an organic compound having a high boiling point such as an unreacted monomer from a floatable polymer in a state of an aqueous slurry.
Generally, polymer latices prepared by emulsion polymerization are coagulated by being in contact with acids or salts, heated and subjected to dehydration and drying, and then collected in a form of powder. According to this process, it is impossible to avoid the disadvantage that the collected product contains various impurities such as additives in the emulsion polymerization, decomposites produced in the cause of the polymerization and remaining unreacted monomers
In many cases these impurities not only affect various processabilities of the polymer, but also generate color and smell, and reduce stability of the polymer. Further they lower the operation stability and value of the product. In addition, when using the polymer as wrapping or package materials for foods, the impurities cause problems in smell and even in health.
As processes for removing not only remaining unreacted monomers of a low boiling point but remaining organic compounds of a high boiling point such as unreacted monomers having a boiling point of not less than 100.degree. C., there are employed a treatment of latex in reduced pressure, a gas-liquid contact treatment of latex, a treatment of slurry by steam stripping or by drying, and the like. However, though in polymerization system of vinyl chloride which has a low boiling point, the remaining monomers can be removed from the slurry of the resulting suspension polymerization by various methods, it seems impossible to remove remaining monomers from polymerization system of monomers having a boiling point of not less than 100.degree. C., as explained hereinbelow.
Namely removal from the latices is not advantageous because foaming of the slurry and scale deposition occur. The steam stripping treatment of the slurry is not economical because of requirement of a large amount of steam, and affects quality of the product because of long contact with steam. Also the drying treatment tends to result in fires and degrades quality of the product due to oxidation.
In case of an MBS resin which is a graft copolymer prepared by polymerizaing 10 to 90 parts (part by weight, hereinafter the same) of one or more monomers selected from a vinyl aromatic compound, a methacrylate, an acrylate and a vinyl cyanide compound in the presence of 10 to 90 parts of a butadiene polymer consisting of 0 to 50% (% by weight, hereinafter the same) of styrene and 50 to 100% of butadiene (the talal amount of the monomers and the butadiene polymer being 100 parts), it is impossible to avoid that the unreacted monomers remain at a high concentration of 1000 ppm (in the resin), because for ensuring its processability and quality the polymerization reaction, is controlled so as not to be completed to the end. When using such an MBS resin as wrapping or package materials for foods, removal of the remaining organic compounds such as the unreacted monomers is essential, because it is strongly desired that the remaining styrene monomer in the MBS resin is removed so that a concentration of the monomer is not more than 10 ppm when using for food wrapping materials.
According to the conventional processes, however, the removal of the organic compounds having a high boiling point to such a desired concentration level is substantially impossible. For instance, when the removal of the remaining monomers is carried out in a continuous process by using an admixing tank, there is often observed a phenomenon that a concentration of the unreacted monomers in a drain slurry increases because the unreacted monomers short-pass at a high concentration.
An object of the present invention is to provide a process for sufficiently removing organic compounds having a high boiling point from an aqueous slurry of a floatable polymer containing the organic compounds.
This and other objects of the present invention will become apparent from the description hereinafter.